The drive for lower cost satellite links has spurred the development of low bit rate voice coding, allowing the transmission of voice over low powered, narrow band carriers. The narrow band carriers must be spaced closely together in the frequency spectrum to effectively utilize satellite power and bandwidth. Consequently, any frequency errors introduced in the terrestrial and satellite hardware can then cause receiving stations substantial difficulty in finding and locking on to the correct narrow band carrier. The frequency error for a ground station attempting to lock to a narrow band carrier may be much greater than the spacing between the narrow band carriers, causing several narrow band carriers to fall within the band of frequency uncertainty. The frequency uncertainty of the received narrow band carrier typically is caused by the following five separate and independent contributing factors:
1. Frequency reference (local timebase) error in the transmitting ground station.
2. IF to RF frequency translation error in the transmitting ground station.
3. Uplink to downlink frequency translation error in the satellite.
4. RF to IF frequency translation error in the receiving ground station.
5. Frequency reference (local timebase) error in the receiving ground station.
Satellite Doppler effects may also contribute, but the contribution is normally negligible with geostationary satellites--the category of satellites to which the present invention is addressed.
The five factors above are briefly discussed in turn. First, the frequency reference (local timebase) and IF to RF frequency translation errors in the transmitting station may be significant, affecting its carrier uniquely and specifically. Second, the uplink to downlink frequency translation error in the satellite has in the past been the dominant factor, amounting to as much as tens of kilohertz. This error affects all the ground stations transmitting through the same satellite, and shows up as a shift of the whole received spectrum. System control stations can now compensate for this shift with reasonable accuracy by measuring the translation shift and using this information to adjust narrow band frequency assignments, or use other means to compensate for this error to reduce the frequency errors in the assigned narrow band carriers as received by each ground station. Third, the RF to IF frequency translation and frequency reference (local timebase) errors in the receiving station may also be significant, affecting all the received narrow band carriers by the same amount.
These five factors combine to create a net error which can cause the desired narrow band carrier to fall outside the bandwidth in which the receiver is searching, thereby preventing the receiving station from locking on to the transmitting station's frequency. In extreme cases, another narrow band carrier with the same symbol rate and modulation type may be present in the bandwidth in which the receiver is searching, causing the receiver to lock to the wrong narrow band carrier. Acquisition performance is measured by (i) the probability of successful acquisition (ii) the risk that the receiver locks on to the wrong narrow band carrier with no automatic means to break lock and resume the search for the correct narrow band carrier, and (iii) how quickly the correct narrow band carrier is acquired. The five error factors enumerated above all must be addressed in order to achieve optimal acquisition performance. Current methods of addressing the above factors require making design trade-offs in certain fixed parameters related to narrow band carrier acquisition, which result in reasonable performance. However, maximum performance cannot be achieved because the same acquisition scheme is being used regardless of the size of the shift caused by the net error. Further, the receiving station must be designed specifically to acquire a narrow band carrier in a spectrum of other narrow band carriers, which prevents the receiving ground station from employing fast, wide band carrier acquisition methods.
Within the prior art, various compensation techniques are disclosed in an attempt to reduce frequency uncertainty or reduce signal acquisition time. As an example, U.S. Pat. No. 5,471,657, issued to Archana M. Gharpuray is directed to a tuning system for satellite ground stations, and a method to compensate for offsets in transmission frequency used by ground stations. A stated object of '657 is to allow "a large number of remote ground stations to compensate for offsets and drift in a bursty satellite communications network without consuming communications bandwidth and without adding to the cost of the remote terminals." The technique of '657 relies on measurement of the relative frequency offset of signals through the satellite, and provides that information from a network control station to remote terminals in the satellite communication network. While this technique does allow the remote terminals to more readily acquire the received signals, '657 does not disclose techniques for rapid signal acquisition in widely varying circumstances. As examples, when there are interfering narrow band signals within the frequency uncertainty, or when the narrow band frequency uncertainty is larger than the narrow band acquisition window.
In another example, U.S. Pat. No. 5,481,561, issued to Russell J. Fang, is directed to techniques for use of CDMA in a DAMA manner to support bursty point-to-point communications between small user communications networks, while conserving bandwidth. The frequency compensation technique of the network control station of '561 uses a precise oscillator with long term stability to send a continuous wave (CW) pilot through the satellite, then watches for the return tone. The network control station then corrects for frequency errors by adjusting the frequency of the transmitted pilot so that the received CW signal is at a precise desired frequency in the center of the downlink frequency band. The remote stations use this corrected pilot for satellite acquisition and tracking. This technique provides for compensation of errors in the satellite, but '561 does not disclose a technique to compensate for errors in the individual uplink and downlink stations, nor does the technique of '561 disclose a technique for rapid acquisition without a pilot tone or with interfering signals within the frequency uncertainty, or when the narrow band frequency uncertainty is larger than the narrow band acquisition window.
As another example, U.S. Pat. No. 5,365,450, issued to Leonard Schuchman et al. is directed to a method to enable rapid and accurate measurement of position, and more particularly to global position system (GPS) precise position location in urban canyon or sight obstructed environments. Included within the specification and disclosure of '450, is a technique to resolve frequency uncertainty in the GPS signals and thereby reduce the time required to develop an accurate position. The frequency measurement from tracking a first satellite is used to calibrate and remove the frequency bias of the GPS local oscillator. This technique allows search of a single frequency cell to achieve rapid subsequent acquisition of other GPS signals. A principal advantage of '450 for GPS acquisition is the use of the frequency bias information from the first satellite to narrow the search band for subsequent channels. This technique of '450 does not disclose a system that is adaptable to satellite communication systems that only require acquisition of a single narrow band channel. The technique of '450 is relevant for satellite systems in non-geosynchronous orbit with significant Doppler components. However, conventional satellite communication systems generally use satellites in geosynchronous orbit with minimal Doppler.
The prior art and known techniques make design trade-offs in the selection of fixed operating parameters or rely on highly precise narrow band carrier frequency control to acquire the correct narrow band carrier. None of the prior art and known techniques provide a dynamically changing acquisition method to adapt to the circumstances of the specific narrow band carrier acquisition provide an ability to adapt or select acquisition methods based on differing conditions.